A compound semiconductor device, an AlGaN/GaN Field effect transistor (FET), has been developed by utilizing a heterojunction of AlGaN/GaN, which is a nitride type compound semiconductor, and using a GaN layer as an electron transit layer. The GaN is a material that provides a wide bandgap, high breakdown electric field strength, and a high saturation electron velocity. Therefore, the GaN is a promising material for a semiconductor device that achieves a high voltage operation, a low on-resistance, and a high output.
Related techniques are discussed in, Japanese Laid-open Patent Publication Nos. 2008-124262, 2001-102565, and 2008-227014.
Generally, in order to achieve a high efficiency switching operation using a transistor, reducing an on-resistance of the transistor is desired.
The on-resistance in the AlGaN/GaN FET will be described by referring to FIG. 1. In the AlGaN/GaN FET, contact-resistances exist at a source electrode S, a drain electrode D, and an AlGaN electron supply layer that is a lower layer of the source electrode S and the drain electrode D. In two-dimensional electron gas (2DEG) generated in an electron transit layer near an interface to the electron supply layer, mainly, gate-source resistance, gate-drain resistance, and a channel resistance below a gate electrode G exist. A sum of the contact resistance, the gate-source resistance, the gate-drain resistance and the channel resistance is substantially equal to the on-resistance.
In the AlGaN/GaN FET, increasing Al composition of the AlGaN layer decreases the channel resistance. On the other hand, higher Al composition makes a barrier higher, thereby contact resistance of the source electrode and the drain electrode increases. The increase of the contact resistance is significant and the on-resistance includes the contact resistance and the channel resistance as the constituents, and the on-resistance is not reduced as a whole, and sometimes may even increase. In this case, a high efficiency operation of the AlGaN/GaN FET is difficult.